The invention relates to a spark plug with creepage spark gap for internal combustion engines.
Such spark plugs with creepage spark gap, which is formed on the surface of the insulator, on the combustion chamber side, between the central electrode and the side electrode, are distinguished by a much lower ignition voltage compared with a spark plug with spark gap in air. It has been found in this respect that the firing voltage is all the lower the greater the di-electric constant of the material of the insulator. However, such an insulator of high-dielectric material results in a spark plug of a relatively high capacitance, which causes a disruptive discharge at the creepage spark gap. Due to the very hot spark, of several ten thousand degrees, produced upon disruptive discharge, the surfaces of the electrodes and, in particular, the slideway of the creepage spark gap are greatly eroded, as a result of which the proper function of the spark plug and its service life are in turn considerably impaired.
In the case of a known spark plug of the type stated at the start (German Patent Application No. P 35 33 123.2), to avoid the breakdown phase with high energy, the insulator is divided transversly in the end region on the combustion chamber side and the larger-volume upper part on the connection side is made of a material having a low-dielectric constant and the smaller-volume lower part on the combustion chamber side is made of a material having a very much higher dielectric constant. However, the total capacitance of the spark plug cannot be kept small enough, so that the energy in occasionally occurring breakdown phases does after all cause notable erosion in the insulator surface.